1. Field of the Invention
This invention relates to a coding apparatus usable in various systems such as facsimile machines.
2. Description of the Prior Art
In general, MR-type (Modified Read type) coding apparatus in facsimile machines determine or select one of different coding modes by using a correlation between conditions of variations in pixels in a reference line and conditions of variations in pixels in a coded line. The reference line immediately precedes the coded line.
In a prior art coding apparatus of such a type, with respect to each of reference lines and coded lines, a detection is given of a point at which an image signal changes from a white pixel level to a black pixel level or from a black pixel level to a white pixel level. Widen such a change point is detected, a coding mode is determined in accordance with the conditions of the change point. The time portion of the image signed which lasts until the moment of the occurrence of the change point is coded according to the determined coding mode. Then, the process of detecting a next change point is restarted for determining a next coding mode. Such a prior art design is disclosed in various documents, for example, Japanese published unexamined patent applications 58-88971, 61-212967, and 61-232776.
The above-mentioned prior art design has a problem in the speed of coding which is caused for the following reason. In the prior art design, when one of the process of detecting a change point in an image signal and the process of determining a coding mode is completed, the other is started. Specifically, while the process of detecting a change point is continued, the process of determining a coding mode remains suspended. In addition, while the process of determining a coding mode is continued, the process of detecting a change point remains suspended. Thus, in the prior art design, the process of detecting a change point and the process of determining a coding mode are executed alternately, so that the coding process tends to be interrupted at a higher frequency and therefore the speed of the coding process tends to be low.